Our main scientific interest and challenge at the πlab is to understand how to utilize light in order to efficiently de-contaminate water, air and surfaces. Within this framework we have studied a variety of aspects related to the use of titanium dioxide as a photocatalyst. Our most significant achievement in the area of photocatalysis is probably the development (together with Prof. Adam Heller, Austin, TX) of the so-called “self- cleaning glass” made of thin, transparent, strongly adhered and highly photoactive TiO2 film, overcoating glass substrates. The main concept there was to obtain relatively high surface area yet without surface corrugation that could have rendered transparency. This was achieved by arresting the sol-gel reaction, leading to the formation of sintered nanocrystallites. No less important was the understanding of the role of sodium migration from the glass to the nascent TiO2 and suggesting ways for preventing its diffusion.
Another achievement in the area of photocatalysis was the finding of long range remote degradation effects, caused by diffusion of oxidizing species away from the photocatalyst surface. For this study, we have introduced the use of chemisorbed self-assembled monolayers (SAMs), located at will at pre-designed distances, as a probe for measuring these remote photodegradation effects.
Being based on the strong oxidation potential of the hydroxyl radicals formed on the photocatalyst surface, TiO2 photocatalysis can be expected to have very low selectivity. This lack of sensitivity to the type of contaminants seems to be benevolent at first glance. However, it also implies that the photocatalyst does not give any priority to highly hazardous contaminants coexisting with organic contaminants of low toxicity. This shortcoming is further aggravated by the fact that while many low-toxicity contaminants can be degraded by biological means, many of the highly hazardous materials are non-biodegradable.
During the last decade we worked hard to convince the community in the need for specific photocatalysis and to introduce new means to achieve specificity beyond the trivial control of adsorption through manipulating the surface’s electric charge. In this context, two approaches were developed and studied. The first method, coined “Adsorb & Shuttle” was based on the construction of robust, immobile organic molecular recognition sites (MRS) on inert substrates, located in the vicinity of the photocatalyst. These covalently bound self-assembled molecules physisorb target molecules in a selective manner. Once physisorbed, the target molecules surface-diffuse from site to site towards the interface between the inert domains and the photocatalytic domains, where they are destroyed. The feasibility of this approach was demonstrated by us in the degradation of 2-methynaphthoquinone, various dyes and diisopropyl methyl phosphonate (DIMP), a nerve gas (sarin) simulant.
A second approach for specificity was to imprint the target contaminants at the surface of the photocatalyst during its preparation, thus affecting not only the rate by which the target molecules are degraded, but also the distribution of their end- products. This approach was demonstrated successfully with DIMP.
Coupling between titanium dioxide and inert supports may affect photocatalysis not only through enhancing adsorption, but also by assisting the separation of photogenerated holes from photogenerated electrons. These phenomena were studied (with Y. Cohen (ChE, Technion)) in systems comprising of TiO2 and carbon nanotubes (CNTs), either embedded within polymeric nanofibers of poly(acrylonitrile) (PAN) made by electrospinning, or within hybrid particles where the CNTs are partially embedded in the particles. Of particular interest is the photocatalytic reduction of the highly toxic Chromium(VI) species and the effect of CNT functionalization on the performance of the nano-composite fibers. Within this framework of composite materials, we have also demonstrated that the common dogma, according to which photocatalytic rates are enhanced whenever charge separation is improved, may be incorrect under certain conditions.
The need to characterize the length of the contour of the different domains within microscopic particles inspired us to develop (together with S. Yaron, (BFE, Technion)) a FRET-based method for that.
During the last three years we are engaged in an effort to study and develop new materials that have prospects to be more efficient in utilizing light. Along this line we have synthesized and characterized a variety of bismuth-containing ternary oxides and have studied their photocatalytic properties, which were found to be very promising.
Our group has shown lasting scientific interest in the area of surface phenomena in semiconductors. We have collaborated successfully with Dan Ritter (EE, Technion) on utilizing self-assembled monolayers chemisorbed on III-V semiconductors to passivate the surface of InGaAs-InP heterojunction bipolar transistors, with which a remarkable decrease in the parasitic current of III-V transistors and diodes was obtained. The extent of decrease in the leakage current was found to depend on the planar direction of the devices’ sidewalls and could be as high as three orders of magnitude. In parallel, we have studied (together with Josef Salzman (EE,Technion) and S. Brandon (ChE, Technion)) the role of mass transport in the photoelectrochemical etching of GaN, and used (together with Nir Tessler (EE, Technion)) photopatternable self-assembled monolayers as templates for polymeric Field Effect Transistors, formed on titanium dioxide.
For many years we have worked intensively with Fourier Transformed Infra Red (FTIR) spectroscopy, gaining expertise in designing home-made accessories and in data analysis. This expertise was found of large value when we collaborated with Ory Ramon (BFE, Technion), Yachin Cohen (ChE, Technion) and later with Yoav Livney (BFE, Technion) on studying the effect of co-solutes (salts and sugars) on the scenario of collapse and phase separation in hydrophobic polymers such as Poly(N-Isopropylacrylamide). Here, we were able to provide a detailed scenario of the phase separation process by precise analysis of the data and by deconvoluting the relevant FTIR peaks, providing molecular level explanations to phenomena that are usually explained by changes in bulk properties (LCST temperature and viscosity, for example). It was this expertise in FTIR, together with our constant willingness to enter new fields that provided the background for a successful collaboration with Y. Zeevi (EE, Technion) on developing an automated procedure for blind separation of mid-infrared spectra comprising of unknown contributions from several unknown sources. The method, demonstrated with diffused reflectance spectra of plants’ leaves, can be utilized for remote sensing as well as for identifying intermediate products and end-products of chemical reactions.